1. Field of the Invention
The present invention relates to a head-mounted visor with variable light transmission. It can be applied notably to helmet-mounted systems for aircraft pilots. More generally it can be applied to protection systems which need to be freed from the constraints of slow variation in transmission, notably due to the absorbent element constituted by photochromic agents.
A helmet-mounted visor for an aircraft pilot generally has two essential protective functions. A first protective function is that of protecting the face against external mechanical forces. This protection is usually provided by an often cylindrical or spherical plate made of transparent thermoplastic material, polycarbonate for example.
A second protective function is that of protecting the eyes against solar radiation, the visor being capable of getting shaded or lightened and hence of absorbing different amounts of light energy in a reversible way, as a function of the ambient luminosity, through the presence of chemical substances integrated into the thickness or surface of the visor. These substances causing variation, as a function of the luminosity, in the coloring of the material that they integrate are called photochromic substances.
More particularly, these photochromic substances are constituted by molecules whose structure gets modified reversibly under the effect of photons that they receive, this modification giving rise to a change in color. Known photochromic substances in use are the groups of substances known as spiropyranes or spiroxazines.
2. Description of the Prior Art
There exist variable transmission visors formed by a transparent thermoplastic plate, the photochromic substance being incorporated during the injection of the part or deposited on the surface by a gun or by molding for example. These visors absorb a part of the incident light energy identically at every point on the visor.
A plastic based on photochromic substances may, in its clear state, provide for transmission of light of about 75% to 85%. In the presence of high luminosity, the plastic attains a darkened state and the transmission may drop to values of 20% to 30% for example. The levels of transmission in the clear state and in the darkened state depend on the nature of the photochromic substances used or on the composition of the mixture of these substances. These levels of transmission also depend on the respective concentrations of photochromic substances in the thickness or on the surface of the visor. At ambient temperature, the reaction times of the photochromic substances are generally lengthy. These reaction times are equal to about one minute for darkening and several minutes for illumination.
For certain required functions, these reaction times are too slow. In particular, to provide ocular protection for pilots against solar radiation, these performance characteristics are quite insufficient for certain types of aircraft.
A pilot needs a clear visor when the luminosity is low and a darkened visor when he encounters conditions of intense illumination, when he passes over the cloud layer for example. A standard photochromic visor as defined here above poorly meets the pilot's requirements. It is in general excessively clear or excessively darkened because of the above-mentioned excessively lengthy reaction times inherent in the very nature of the photochromic substances: it is notably the change from the darkened state to the clear state that occurs with far too great a delay. This has the consequence of troubling the pilot's vision and hence of reducing his safety.
There are of course known ways of using LCD (liquid-crystal display) type solutions wherein the reaction times are short. However, these solutions, which can be well integrated with glass are very difficult to integrate with plastics such as those used in visors.